Cell Division: Prokaryotes vs. Eukaryotes

2. Cell division in prokaryotes takes place in two stages The DNA is replicated The cell elongates, then splits into two daughter cells The process is called binary fission 7.1 Prokaryotes Have aSimple Cell Cycle

3. Fig. 7.1 Cell division in prokaryotes

4. Cell division in eukaryotes is more complex than in prokaryotes because 1. Eukaryotic contain far more DNA and a nucleus 2. Eukaryotic DNA is packaged differently It is in linear chromosomes compacted with proteins 7.2 Eukaryotes Have aComplex Cell Cycle

5. Eukaryotic cells divide in one of two ways Mitosis Occurs in somatic (non-reproductive) cells Meiosis Occurs in germ (reproductive) cells Results in the production of gametes 7.2 Eukaryotes Have aComplex Cell Cycle

6. The complex cell cycle of eukaryotic cell is composed of several stages Interphase • G1 phase • Primary growth phase • S phase • DNA replication • G2 phase • Microtubule synthesis, growth, mitochondria replication • M phase • Chromosomes pull apart • C phase • Cytoplasm divides Mitosis Cytokinesis

7. Fig. 7.2 How the cell cycle works

8. 7.3 Chromosomes • Chromosomes were first observed by the German embryologist Walther Fleming in 1882 • The number of chromosomes varies enormously from species to species • The Australian ant Myrmecia spp. has only 1 pair • Some ferns have more than 500 pairs • Chromosomes exist in somatic cells as pairs • Homologous chromosomes or homologues

9. Fig. 7.3 • Replicated chromosomes consist of two sister chromatids • These are held together at the centromere • Diploid cells have two copies of each chromosomes

10. Fig. 7.4 7.3 Chromosomes • Humans have 46 chromosomes The 23 pairs of homologous chromosomes can be organized by size This display is termed a karyotype

11. 7.3 Chromosomes • Chromosomes are composed of chromatin • Complex of DNA (~ 40%) and proteins (~ 60%) • A typical human chromosome contains about 140 million nucleotides in its DNA • This is equivalent to • About 5 cm in stretched length • 2,000 printed books of 1,000 pages each! • In the cell, however, the DNA is coiled

12. Interphase (“in-between divisions”) • Cell grows (G1), replicates DNA (S), and grows some more (G2) • Mitosis • Prophase (“prepares”) • Nuclear envelope breaks down • Chromosomes condense further • Spindle apparatus is formed • Metaphase (“middle”) • Chromosomes align along the equatorial plane • Spindle fibers attach to the chromosomes

13. Fig. 7.7

14. Anaphase (“away”) • Sister chromatids separate • They are drawn to opposite poles by shortening of the microtubules attached to them • Telophase (“tears into two”) • Nuclear envelope reappears • Chromosomes decondense • Spindle apparatus is disassembled • Cytokinesis • Two diploid daughter cells form • Mitosis

15. Fig. 7.7

16. Fig. 7.8 Animal cells • Cytokinesis • Cleavage furrowforms, pinching the cell in two Plant cells • Cell plateforms, dividing the cell in two

17. Fig. 7.10 7.5 Controlling the Cell Cycle • The eukaryotic cell cycle is controlled by feedback at three checkpoints

18. Fig. 7.13 7.6 What is Cancer? • Cancer is unrestrained cell growth and division • The result is a cluster of cells termed a tumor • Benign tumors • Encapsulated and noninvasive • Malignant tumors • Not encapsulated and invasive • Can undergo metastasis • Leave the tumor and spread throughout the body

19. 7.6 What is Cancer? • Most cancers result from mutations in growth-regulating genes • There are two general classes of these genes • 1. Proto-oncogenes • Encode proteins that simulate cell division • If mutated, they become oncogenes • 2. Tumor-suppressor genes • Encode proteins that inhibit cell division • Cancer can be caused by chemicals, radiation or even some viruses

20. The p53gene plays a key role in the G1 checkpoint of cell division The p53 protein (the gene’s product), monitors the integrity of DNA If DNA is damaged, the protein halts cell division and stimulates repair enzymes If the p53 gene is mutated Cancerous cells repeatedly divide No stopping at the G1 checkpoint 7.7 Cancer and Controlof the Cell Cycle

21. Fig. 7.14 Cell division and p53 protein

22. 7.8 Curing Cancer • Potential cancer therapies are being developed to target seven different stages in the cancer process • Stages 1-6 • Prevent the start of cancer within cells • Focus on the decision-making process to divide • Stage 7 • Act outside cancer cells • Prevents tumors from growing and spreading

23. Fig. 7.15 New molecular therapies for cancer Stopping tumor growth Receiving the signal to divide Stepping on the gas Passing the signal via a relay switch Amplifying the signal Releasing the “brake” Checking that everything is ready

24. 7.9 Discovery of Meiosis • Meiosis was first observed by the Belgian cytologist Pierre-Joseph van Beneden in 1887 • Gametes (eggs and sperm) contain half the complement of chromosomes found in other cells • The fusion of gametes is called fertilization or syngamy • It creates the zygote, which contains two copies of each chromosome

25. Fig. 7.16 Contain one set of chromosomes • Sexual reproduction • Involves the alternation of meiosis and fertilization • Asexual reproduction • Does not involve fertilization Contains two sets of chromosomes

26. 7.10 The Sexual Life Cycle • The life cycles of all sexually-reproducing organisms follows the same basic pattern • Haploid cells or organisms alternate with diploid cells or organisms • There are three basic types of sexual life cycles

27. Fig. 7.18 Three types of sexual life cycles

28. Fig. 7.19 The sexual life cycle in animals

29. 7.11 The Stages of Meiosis • Meiosis consists of two successive divisions, but only one DNA replication • Meiosis I • Separates the two versions of each chromosome • Meiosis II • Separates the two sister chromatids of each chromosome • Meiosis halves the number of chromosomes

30. Haploid gametes PROPHASE I Germ-line cell II TELOPHASE METAPHASE I II II I ANAPHASE II I Fig. 7.22 How meiosis works Diploid cell

31. Meiosis I • Prophase I • Homologous chromosomes pair up and exchange segments • Metaphase I • Homologous chromosome pairs align at random in the equatorial plane • Anaphase I • Homologous chromosomes separate and move to opposite poles • Telophase I • Individual chromosomes gather together at each of the two poles

32. Fig. 7.20 • The longest and most complex stage of meiosis • Homologous chromosomes undergo synapsis • Pair up along their lengths • Meiosis I • Prophase I • Crossing overoccurs

33. Interkinesis Fig. 7.23

34. Meiosis II • After meiosis I there is a brief interphase • No DNA synthesis occurs • Meiosis II is similar to mitosis, but with two main differences • 1. Haploid set of chromosomes • 2. Sister chromatids are not identical

35. Prophase II • Brief and simple, unlike prophase I • Metaphase II • Spindle fibers bind to both sides of the centromere • Anaphase II • Spindle fibers contract, splitting the centromeres • Sister chromatids move to opposite poles • Telophase I • Nuclear envelope reforms around four sets of daughter chromosomes • Meiosis II

36. No two cells are alike Fig. 7.23

37. 7.12 Comparing Meiosis and Mitosis • Meiosis and mitosis have much in common • However, meiosis has two unique features • 1. Synapsis • Homologous chromosomes pair all along their lengths in meiosis I • 2. Reduction division • There is no chromosome duplication between the two meiotic divisions • This produces haploid gametes

38. Fig. 7.24 Unique features of meiosis

39. Fig. 7.25 A comparison of meiosis and mitosis

40. Sexual reproduction increases genetic diversity through three key mechanisms 1. Independent assortment 2. Crossing over 3. Random fertilization 7.13 Evolutionary Consequencesof Sex

41. Fig. 7.26 Independent assortment • In humans, a gamete receives one homologue of each of the 23 chromosomes • Humans have 23 pairs of chromosomes • 223 combinations in an egg or sperm • 8,388,608 possible kinds of gametes Three chromosome pairs 23 combinations

42. Fig. 7.20 Crossing over • DNA exchanges between maternal and paternal chromatid pairs This adds even more recombination to independent assortment that occurs later

43. Random fertilization • The zygote is formed by the union of two independently-produced gametes • Therefore, the possible combinations in an offspring • 8,388,608 X 8,388,608 = • 70,368,744,177,664 • More than 70 trillion! • And this number does not count crossing-over

44. Importance of Generating Diversity • Genetic diversity is the raw material that fuels evolution • And no genetic process generates diversity more quickly than sexual reproduction